The present invention is directed toward an ophthalmic device comprising superposed pairs of particular light polarizing elements; one of such pairs being fixedly retained in a frame member while the elements of the other such pair are rotatable.
The concept of providing variable light transmission characteristics to an ophthalmic device by employing light polarizing elements which may be rotated with respect to each other is not itself new. For background information on systems of this type, reference should be had to U.S. Pat. No. 2,005,246 issued to Edwin H. Land on June 18, 1935; U.S. Pat. No. 2,251,330 issued to Murray N. Fairbank on Aug. 5, 1941; and U.S. Pat. No. 2,565,362 issued to Vaito K. Eloranta on Aug. 21, 1951. See also copending and now allowed U.S. patent application Ser. No. 608,401, U.S. Pat. No. 4,119,369, filed on Aug. 27, 1975 in the names of Eloranta et al. which describes a particular arrangement of light polarizing elements in an ophthalmic device to provide variable density light polarizing functionality.
The most widely used type of synthetic light polarizer is the polyvinyl alcohol-iodine complex polarizer. It consists of linear polyiodides contained within the polyvinyl alcohol helix. By orienting the polyvinyl alcohol matrix unidirectionally the transmission moments of the absorbers are also so oriented and the material becomes dichroic.
The manufacture of iodine stained dichroic light polarizing elements, which involves stretching polyvinyl alcohol and subsequently dyeing the material with a dichroic stain, iodine, is well known and is disclosed, for example, in U.S. Pat. No. 2,237,567 of Edwin H. Land issued on Apr. 8, 1941. In accordance with that patent it is disclosed that a cast sheet of polyvinyl alcohol is first formed from an aqueous solution of the material. The dried cast sheet is then heated to a temperature at which it can be extended by stretching, preferably in a moist atmosphere. It is further disclosed in that patent that the stretched sheet may be bonded to a protective layer after the stretched sheet is cooled. Dichroic stain may be applied to one or both surfaces of the stretched sheet.
In a later patent issued to Alexander Thomas on May 15, 1945, U.S. Pat. No. 2,375,963 an improvement in the process of making an iodine stained polarizer is described and essentially comprises washing the polarizer after the step involving staining with iodine. This results in removing uncombined iodine and forming a more stable product.
A further improvement in the above light polarizing element is explained in U.S. Pat. No. Re. 23,297 issued on Nov. 28, 1950 to Mark Hyman, Jr. et al. That improvement comprises a protective surface layer on the iodine stained polyvinyl alcohol polarizer. That surface layer comprises an ester of polyvinyl alcohol, the ester being of a polybasic acid or a derivative of such an acid, particularly an inorganic polybasic acid, and more particularly boric acid, thereby providing a polyvinyl borate. The borating step is disclosed to provide greatly improved stability to the light polarizer not only against heat but also against moisture and ultra violet radiation. It is accomplished by treating the stained polarizing element with a boric acid solution. Specifically the ester formed on the surface of the light polarizer is believed to be polyvinylorthoborate.
A further improvement in the preparation of iodine stained polarizers is described in U.S. Patent Application Ser. No. 810,996 of Norman Schuler filed on June 29, 1977 and now abandoned in favor of copending continuation application, U.S. Ser. No. 900,728, filed Apr. 27, 1978. That application describes adding a zinc salt and potassium iodide to the borating solution in order to provide good blue absorption and stabilize the dichromophore responsible for red light absorption. An iodine stained polarizer as described and preferably one manufactured with zinc ion in borating solution as described above is employed in the present invention.
In order to produce light polarizing ophthalmic devices which may be employed, for instance, as variable density sunglasses, cautions must be observed to prevent the user from accidently rotating the absorption axis of the rotatable polarizer into the normal position with respect to the absorption axis of the fixed polarizer which would produce substantial light extinction; that is the viewer would suddenly be thrust into a blind condition. It has been found that for driving safety the density of sunglasses should not go below about 6 to 10 percent transmission and preferably should go no lower than about 8 percent transmission. Furthermore in order to provide effective sunglass functionality when the absorption axes of the rotatable and fixed light polorizers are substantially parallel the light transmission of the "open" system should be on the order of 35 to 40 percent. All this is desirably accomplished while maintaining the primary functionality of light polarizing ophthalmic devices, that is reducing specular glare. Preferably specular glare is to be held to within about 1 percent leakage in all attitudes of rotation of the rotatable lens.
Given these considerations it appears evident that in a system even where a full 90.degree. of rotation is available for rotatable lenses, among effects which will be achieved when the polarizers are in the crossed position, that is when the absorption axes of the fixed and rotatable lenses are normal to one another, the transmission of incident radiation through the pair will be on the order of about 8 percent. Unfortunately as the rotatable lens is rotated through an arc of less than 90.degree., from the crossed position e.g., through an arc on the order of about 60.degree., it is found that light transmission through the lenses is too low to be effectively employed in a commercial variable density sunglass product. In other words, a higher degree of rotation is required in order to get the respective absorption axes sufficiently close to a parallel position to give effective "open" transmissivity of over about 35 percent.
For a recitation of the manufacturing details of iodine stained polarizers useful in the present invention reference should be had to the aforementioned copending application Ser. No. 900,728. The light transmissivity of an individual light polarizing element is governed by the amount of iodine absorbed by the polyvinyl alcohol matrix which is in turn a function of the residence time of the polyvinyl alcohol film in the iodine bath and the concentration of the bath. By adjusting residence time and/or concentration, various densities or light transmissivity characteristics of polarizers can be achieved. Taking the obvious approach to producing polarizers for employment in the present invention, elements with a high initial transmissivity on the order of about 48 percent were prepared and found to permit approximately 8 percent light leakage or transmissivity when their absorption axes are in the cross position. Unfortunately, however, when used in a rotatable system in which the degree of rotation of the rotatable lens is held to on the order of about 60.degree., only about 30 percent transmission in the full open position could be obtained. Likewise it was found that with polarizers of such high transmissivity in the crossed position more than 5 percent glare leakage occurred with about 3/4 of a percent glare leakage in the open position; 5 percent glare leakage being unacceptable. Open position glare leakage is lower than closed position glare leakage due to the fact that in the open position both absorption axes, that is the absorption axis of the rotatable and fixed lenses, are generally horizontal and near parallel, thereby fortifying each other and substantially eliminating specular glare. However, in the closed position the absorption axis of the rotatable lens closer to a vertical attitude with respect to the horizontal absorption axis of the fixed polarizer and accordingly cannot aid in glare removal.
With a higher density, for example, a polarizer which passed only 38 percent of incident light, it was found that glare leakage is below 1 percent. However, "open" or parallel axes transmissivity of the lens pair is under 30 percent making it generally unacceptable in a commercial product. Various empirical experiments have shown that maximum transmissivity in a variable density light transmitting ophthalmic device, when the respective absorption axes are angularly related to each other by 60.degree. plus the angle between the absorption axes which results in 8 percent transmission, is achieved when the superposed lenses comprise polarizers having individual transmissivities on the order 45.3 percent. The obvious conclusions that such a polarizer would produce optimal results when employed in a system of the type described have turned out to be fallacious as unacceptable parallel position glare leakage results. It has been discovered that if one selects lens pairs with a single lens transmissivity less than 45.3 percent, on the order of 42-44 percent, a parallel axes lens pair transmissivity of between over 35 percent results. The lenses comprising such polarizers will provide, in all attitudes, glare reduction of less than about 1 percent and when the rotatable lens is rotated about 60.degree. from the 8 percent transmission position, nearly maximum transmittance is obtained with these polarizers.